In the present invention, we are concerned with axial flux permanent magnet machines. Broadly speaking these have disc- or ring-shaped rotor and stator structures arranged about an axis. Typically the stator comprises a set of coils each parallel to the axis and the rotor bears a set of permanent magnets and is mounted on a bearing so that it can rotate about the axis driven by fields from the stator coils. FIG. 1a shows the general configuration of an axial flux machine of the present invention with a pair of rotors R1, R2 to either side of a stator S—although a simple structure of the present invention could omit one of the rotors. As can be seen there is an air gap G between a rotor and a stator and in an axial flux machine the direction of flux through the air gap is substantially axial.
There are various configurations of axial flux permanent magnet machine depending upon the arrangement of north and south poles on the rotors. FIG. 1b illustrates the basic configurations of a Torus NS machine, a Torus NN machine (which has a thicker yoke because the NN pole arrangement requires flux to flow through the thickness of the yoke), and a YASA (Yokeless and Segmented Armature) topology. The illustration of the YASA topology shows cross-sections through two coils, the cross-hatched area showing the windings around each coil. As can be appreciated, dispensing with the stator yoke provides a substantial saving in weight and iron losses, but one drawback is loss of rigid structure in which a bearing can be mounted to support rotors. Thus preferably for a YASA topology of double rotor, single stator axial flux motor a bearing is mounted within the stator confines and magnetic forces from rotors on either side of the stator are axially balanced. Rotors being designed to resist bending towards the stator.
For such motors, the air gap between rotor and stator for axial flux motors is small typically of the order of 1 mm. Smaller gaps lead to higher motor torque and power output and is seen as beneficial.
However as said rotor to stator air gap becomes smaller so manufacturing stack-up tolerances derived from stator and rotor components can lead to variable air gap from motor to motor and hence variable output characteristics. In the limit there is possibility of interference of rotor on stator and hence rotor not being able to rotate and a motor being inoperable or catastrophic motor failure if interference occurs during use.
To overcome the problem of stack-up tolerance for double rotor, single stator, axial flux motors, WO 2010/092402 teaches a two stage rotor arranged one at either end of the stator bars, with two air gaps between the ends of the bars and the rotor stages, an annular housing retaining and mounting the stator; a bearing between the rotor and stator. The rotor is thus not otherwise supported in or on the housing and air gaps are set by stator width, bearing length and bearing support faces on rotors. This arrangement provides excellent repeatability of the air gap in motor production, but the teaching of WO 2010/092402 does not apply if a bearing is not placed between stator and rotor but instead bearings are supplied by adjacent machines as is the case for many machine tools and hybrid designs. It is always preferable for multiple connected rotating machines to be correctly aligned, but axial flux motors are particularly sensitive to axial misalignment and/or displacement of rotor to stator which affect's airgaps and so efficiency and in the limit may prevent rotation or cause catastrophic failure.
A frameless torque motor is described by Vogt US 2006/0145566 for securing the position of a radial motor rotor relative to the stator, in which a spacer is included in the air gap. However does not address the issue of how stator to rotor axial alignment is achieved when said torque motor is integrated in to another machine. A possible reason for this is that axial misalignment of a radial flux rotor with its stator minimally affects motor operation whereas axial misalignment of an axial flux motor is of great significance in the limit preventing motor operation or leading to catastrophic failure.
Nielson U.S. Pat. No. 3,719,988 teaches spacer elements for centering of rotors within stators for radial machines when in manufacture or in repair. Centering spacer elements enable fixing of the rotor shaft into bearings within the motor housing. Because motor bearings are not available in axial flux machines, this significantly increases the complexity of alignment.
Oudet in U.S. Pat. No. 5,003,686 teaches the use of spacers in a double stator, single rotor, axial flux electric motor in which spacers are used to separate a rotor part from first and second stator parts. With the rotor part correctly in position the rotor part is stuck (adhesively) or fixed by another means to the main rotor assembly and the stator is similarly fixed in position. This arrangement works for an internally journaled rotor, but does not address adjustment of stator around a fixed rotor and does not solve rotor and stator positioning for an externally journaled rotor shaft. Thus Oudet teaches rough axial placement of the rotor shaft and rotor support using a circlip and shims and fine alignment by sandwiching the rotor part between stator parts, keeping an air gap using two spacers and fixing or (adhesively) sticking rotor and stator components when placed, thereby maintaining their relative positions.
Approaches taken in the prior art do not address the difficulty of shipping and integration of an axial flux machine having no internal bearings, nor of easing disassembly for safe removal and shipment from the field.
We have therefore appreciated the need for an improved assembly method of axial flux machines comprising a simplified means of assembly wherein bearings for the rotor are provided by adjacent machines or offset journals.